The invention relates to an insertion end for a rotary or percussively driven tool such as a chisel boring tool, chisel or a cutting core bit for working rock, concrete or masonry.
Conventionally a rotary or percussively driven tool has an insertion end extending longitudinally along an axis of a rotary or percussive hand tool machine. The interface between the insertion end of the tool and the tool holder of the hand tool machine must be compatible within a specific performance class to provide options for the use of a wide variety of tools. The internationally most widely used standardized insertion ends and associated tool holders, which are disclosed in DE2551125A1 and DE 3 716 915 A1,have a tool-side cylindrical sleeve-shaped guide surface oriented in the direction of the free leading end axially closed locking groove and towards the free leading end axially open trapezoidal rotary driving groove, wherein at least one radially displaceable locking element of the associated tool holder engages in a locking groove and can restrict the axial mobility of the tool in the tool holder.
The practically standardized insertion end and tool holder according to DE22551125A1 have a guide diameter of 10 mm, whereby each have precisely two identical, diametrically opposed locking grooves and rotary driving grooves, which are disposed symmetrically on the circumference. A guide surface, which does not contributed to torque transfer, extending up to the tool-side end of the insertion end communicates with the slightly longer rotary driving groove. These insertion ends were originally designed for a bit diameter of up to 17 mm and are consequently grouped in the range of the small, lower-power percussive drills with a power of less than 650 W. The increasingly higher output hand tool machines, in particular the percussion drilling machines such as hammer drills, however, make it possible to transmit high torques to the tool in certain operating modes. An extension of the practical range of application of these percussion drilling machines has resulted in a drill diameter of 30 mm. Furthermore, when removing the tool from the work piece, in particular in tools stuck in the bore hole, high torques are brought to bear on the tool by the user by virtue of the hand tool locking up. It has been shown, that the drill diameter of more than 17 mm has an increasing tendency to damage; for example, increasing the tendency of the insertion end to break in the zone of the locking groove and to be destroyed within the tool holder. These breakages are more bothersome when the broken end remains inside the percussion drill and can only be removed by dismantling the front part of the percussion drill from the tool holder. Even when there is no breakage when utilizing drills of greater drill diameters, there is a plastic deformation at the insertion end, which results in a disproportionately high wear on the tool holder.
The standardized insertion ends and tool holders disclosed in DE 3 716 915 A1 have a guide diameter of 18 mm, whereby precisely two identical, diametrically opposed locking grooves are present and exactly one rotary driving groove is arranged in one section half of these grooves and precisely two rotary driving grooves are symmetrically arranged in the other section half of these grooves. These insertion ends are designed for higher performance, larger percussion drills and the transmission of greater torques, whereby the problems mentioned in the above paragraphs occurs at higher power classes or torques. Tools with a guide diameter of 18 mm having a substantially smaller drill diameter of 14 mm, however, have poor impact pulse transmission. Furthermore, such disproportional tools are not economical to manufacture.
The resulting loads have the following composition: On the one hand, there is a loading of the insertion end by virtue of the percussive energy of the percussion drill; and on the other hand, there is, a torsion load emanating from the rotary wedges of the tool holder by virtue of the torque generated at the cutting edge. The torsion load transmits to the rotary driving slots of the insertion end. The torque loading is particularly high when there is a wedging of the cutting edge in a drilling reinforcement.
An additional load occurs when the user attempts to withdraw the percussion drill that is exerted by the locking element on the axial locking end of the locking groove and acts upon an at-risk, posterior cross-section of the locking groove. Many years of experience have shown that the cross-section situated in the zone of the axial locking end is especially at-risk by virtue of these combined, multiple-axis loads. The breakdown-mechanical is due to the locally pronounced, multiple-axis stress condition on the axial locking end, which effects a local stiffening via the transverse contraction. The transverse contraction represents a preferred fissure initiator and limits the fatigue strength of the alternately loaded insertion end.
According to DE 4 338 818, an insertion end of larger diameter is received in a tool holder. The tool holder can also receive an insertion end of smaller diameter. The tool holder has extra rotary driving grooves and locking grooves. The cross-section, which is reduced extremely in the axial region, has a poor impact pulse transmission and a low breaking strength, as already mentioned above.